Mobile Bug

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This handy, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cell phone from a distance of 8-10 meters. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for Spying and unauthorized video transmission. The circuit can detect the incoming and outgoing calls, SMS and video transmission. The moment the Bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.



1.R1 ________2.2M

2.R2 ________100K

3.R3 ________2.2M

4.R4 ________1K




7.C1 ________22P

8.C2 ________22P

9.C3 ________0.22 µF

10.C4 ________100 µF


12.C6 _________0.1 µF

13.C7_________ 0.1 µF

14.C8_________ 0.01 µF

15.C9__________4.7 µF

16.IC CA3130

17.IC NE555

18.T1 BC548






Weekly Reports

Week 1

Have made the RF sensor circuit but we are having problems with it. So we are trying out by changing lead lenght and distance between leads to sort it out. We have got the way to increase the range to a radius of 10 metres but presently we are working to implement the basic circuit.

Week 2

Figured out the way to increase the range of the prototype. But still we are having problems with the circuit. Examined the circuit and i think there is a problem with the transistor.So i need help regarding testing a transistor.

Week 3

This is the link for the circuit we r following "mobile _bug"

IC1 functions as a current to voltage converter, since it converts the tiny current released by the 0.22 capacitor as output voltage.

At power on output go high and LED lights for a short period. This is because + input gets more voltage than the – input. After a few seconds, output goes low because the output current passes to the – input through R2.

But there is a problem. i tested the current across the capacitor in idle as well as we my mobile was activated Surprisingly it did not change. So there is some problem with RF detection. capacitor (C3) captures the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency as stated on the web.

IT wud be nice if u suggest some other ways of detecting RF signals.

What makes me think that rest of the circuit id okay is that when the power is switched on IC1 will give a high output and T1 conducts to trigger LED and Buzzer and this is happening in the case of ours.

So in short we require help regarding RF detection.

Work Done

Here is the basic circuit of the mobile bug.

Mob bug.jpg

R3 has been changed to 1M as per convenience.

Component Description

Pin configuration of CA3130



This IC is a 15 MHz BiMOS Operational amplifier with MOSFET inputs and Bipolar output. The inputs contain MOSFET transistors to provide very high input impedance and very low input current as low as 10pA. It has high speed of performance and suitable for low input current applications. Its basic purpose here is to give a low amount of output current when there is a voltage difference between its inputs.

Pin configuration of NE555 Timer


Working of Mobile Bug

Mobile phone uses RF with a wavelength of 30cm at 872 to 2170 MHz. That is the signal is high frequency with huge energy. When the mobile phone is active, it transmits the signal in the form of sine wave which passes through the space. The encoded audio/video signal contains electromagnetic radiation which is picked up by the receiver in the base station. Mobile phone system is referred to as “Cellular Telephone system” because the coverage area is divided into “cells” each of which has a base station. The transmitter power of the modern 2G antenna in the base station is 20-100 watts.

AM Radio uses frequencies between 180 kHz and 1.6 MHz. FM radio uses 88 to 180 MHz. TV uses 470 to 854 MHz. Waves at higher frequencies but with in the RF region is called Micro waves. Mobile phone uses high frequency RF wave in the micro wave region carrying huge amount of electromagnetic energy. That is why burning sensation develops in the ear if the mobile is used for a long period. Just like a micro wave oven, mobile phone is ‘cooking’ the tissues in the ear. RF radiation from the phone causes oscillation of polar molecules like water in the tissues. This generates heat through friction just like the principle of microwave oven. The strongest radiation from the mobile phone is about 2 watts which can make connection with a base station located 2 to 3 km away.

Ordinary LC (Coil-Capacitor) circuits are used to detect low frequency radiation in the AM and FM bands. The tuned tank circuit having a coil and a variable capacitor retrieve the signal from the carrier wave. But such LC circuits cannot detect high frequency waves near the microwave region. Hence in the circuit, a capacitor is used to detect RF from mobile phone considering that, a capacitor can store energy even from an outside source and oscillate like LC circuit.


The non polarized disc capacitor is used to pass AC and not DC. Capacitor can store energy and pass AC signals during discharge. 0.22 capacitor is selected because it is a low value one and has large surface area to accept energy from the mobile radiation. To detect the signal, the sensor part should be like an aerial. So the capacitor is arranged as a mini loop aerial (similar to the dipole antenna used in TV).In short with this arrangement, the capacitor works like an air core coil with ability to oscillate and discharge current.

One lead of the capacitor gets DC from the positive rail and the other lead goes to the negative input of IC1. So the capacitor gets energy for storage. This energy is applied to the inputs of IC1 so that the inputs of IC are almost balanced with 1.4 volts. In this state output is zero. But at any time IC can give a high output if a small current is induced to its inputs. There a natural electromagnetic field around the capacitor caused by the 50Hz from electrical wiring. When the mobile phone radiates high energy pulsations, capacitor oscillates and release energy in the inputs of IC. This oscillation is indicated by the flashing of the LED and beeping of Buzzer. In short, capacitor carries energy and is in an electromagnetic field. So a slight change in field caused by the RF from phone will disturb the field and forces the capacitor to release energy.



IC1 is designed as a differential amplifier Non inverting input is connected to the potential divider R1, R2. Capacitor C2 keeps the non inverting input signal stable for easy swing to + or – R3 is the feedback resistor IC1 functions as a current to voltage converter, since it converts the tiny current released by the 0.22 capacitor as output voltage. At power on LED glows for a moment as IC1 gives a small ouput due to difference in voltage caused by the capacitor which is on the process of charging As capacitor C1 charges both the inputs of the IC1 get equal voltage and thus output current is 0

When the high frequency radiation from the mobile phone is sensed by the circuit, 0.22 cap discharges its stored current to the + input of IC1 and its output goes high momentarily. (in the standby state, output of the differential amplifier is low since both inputs get equal voltage of 0.5 volts or more). Any increase in voltage at + input will change the output state to high.



Normally IC1 is off. So IC2 will be also off. When the power is switched on, as stated above, IC1 will give a high output and T1 conducts to trigger LED and Buzzer .This can be a good indication for the working of the circuit. An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm.

So a circuit detecting gigahertz signals is required for a mobilebug. Here the circuit uses a 0.22?F disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.

Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal.

Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1

This will upset the balanced input of IC1 and convert the current into the corresponding output voltage.

Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4.

Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 0 and ‘null’ inputs (pin 1) of IC1(can be done away with).

When the mobile phone signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This is then amplified by a transistor and thus triggers monostable timer IC2 through capacitor C7.


""RANGE" The prototype version has only limited range of 1 -1.5 meters. But if a preamplifier stage using JFET or MOSFET transistor is used as an interface between the capacitor and IC, range can be increased.


Here is the video link for our project mobile detector

Project By

Avilash Kumar


Animesh Swain